The invention relates to an electrical capacitor consisting of a consolidated stack of mutually layered dielectric plies each of which is provided with a metal layer as a coating. The coatings alternately extend from ply to ply up to the ends of two projections which have arisen by means of an incision which proceeds in the direction of the thickness of the stack and approximately in the center of one side thereof. Furthermore, the surfaces formed in the consolidated stack by the ends of the projections are provided with metal layers serving for contacting. The metal layers connect the antipolar coatings to one another.
The invention further relates to a method for the manufacture of such an electrical capacitor, wherein tapes of synthetic plastic metallized on one side are layered on a drum as dielectric plies, particularly having a wave cut at the edge side, to form an originating capacitor. The originating capacitor is divided into the desired individual capacitors perpendicular to the layer planes, whereby capacitatively ineffective intermediate plies are disposed under given conditions on a plurality of dielectric plies and coatings which respectively form a mother capacitor. The capacitatively effective dielectric plies with the coatings for the next mother capacitor are in turn disposed on the intermediate plies. The originating capacitor which has arisen in such manner is provided with frontal contact layers and is subsequently divided in the region of the intermediate plies and in a direction perpendicular thereto.
The periodical "IBM Technical Disclosure Bulletin", Vol. 25, No. 10, March 1983, pages 5366 and 5367, incorporated herein by reference, discloses a multi-layer capacitor with ceramic as the dielectric which comprises the features of the above capacitor. The manufacture of such ceramic multi-layer chip capacitors is disclosed, for example, in U.S. Pat. No. 3,740,624, incorporated herein by reference, and comprises metal spots applied in a specific arrangement on films of pulverized ceramic material and plastic as a binding agent, these films being subsequently stacked on top of one another such that the metal spots are offset relative to one another from layer to layer, and end at different sides in the finished capacitor. The individual capacitors are punched from a compacted, larger stack of such films and are then subjected to the ceramic sintering firing. The introduction of an incision given such ceramic multi-layer capacitors presents considerable technical difficulties since, if these incisions are already produced when the individual capacitors are punched from the film stack, irregularities in the structure can occur during the following sintering or when these incisions are produced at the finish-sintered ceramic stacks. This means a considerable expense for precision instruments because, in particular, very small capacitors are difficult to manipulate, particularly since a considerable reject rate due to breakage must be feared.
Nonetheless, the known capacitors are not without interest as seen in terms of their structure because the specific arrangement of the individual coatings results in a current conduction given which the currents mutually compensate on opposed coatings so that the capacitor has a low-inductance structure overall.
Low-inductance ceramic chip capacitors are disclosed in the periodical "IBM Technical Disclosure Bulletin", Vol. 24, No. 18, June 1981, pages 437 through 440, incorporated herein by reference. Given these capacitors, the power leads to the antipolar coatings are likewise effected from only one side. This is possible given ceramic chip capacitors because the thickness of the ceramic dielectric layers is greater than 20 .mu.m and the thickness of the metal coatings amounts to at least 1 .mu.m. Nonetheless, the method cannot be implemented on a large industrial scale or can only be thus implemented with considerable technical expense.
On the other hand, stack or layer capacitors having plastic films as dielectric are known, these being manufactured according to the method initially specified (see, for example, German Letters Pat. No. 1,764,541, corresponding to U.S. Pat. Nos. 3,670,378 and 3,728,765, incorporated herein by reference. It is thus a matter of mass-produced products having daily production numbers of more than 1 million items.
The drum winding method (wheel winding method) disclosed in the Letters Patent is schematically presented in the Siemens brochure "Ideal fur Leiterplatten: MK-Schichtkondensatoren" No. B 21/1210, WS 107520, i.e. 1975 edition, pages 6 and 7, incorporated herein by reference. The capacitors resulting there are shown on pages 4 and 5. The right-hand figure on page 4 practically corresponds to FIGS. 5 and 6 of the German Letters Pat. No. 1,764,541, incorporated herein by reference. See also the English language edition of this Siemens brochure: "Ideally Suitable for PC Board Mounting: Metallized Plastic Layer Capacitors", No. B 21/1210.101, WS 3766, incorporated herein by reference.
These capacitors are regenerable, i.e. the metal coatings are so thin that, given a disruptive breakdown, they evaporate around the breakdown location due to the energy thus released, and thus form an insulation region, as schematically shown on page 3 in the cited brochure The regenerability is also disclosed in the German Letters Pat. No. 832,640 (corresponding to Great Britain Letters Pat. No. 686,293), incorporated herein by reference.
The manufacturing method for layer capacitors is also schematically presented in the Siemens brochure "MK-Schichtkondensatoren nun auch mit Polyester-Dielektrikum", No. B 1687, WS 37725, i.e. 1977, page 5. The English language edition of this Siemens brochure, "Metallized Plastic Layer Capacitors now also with Polyester Dielectric", has the number B 1687.101, WS 37712, i.e. was published in 1977. Both documents are incorporated herein by reference.
For employment in printed circuits, it is necessary that the power lead elements of these capacitors be disposed in grid dimensions, i.e. in a whole multiple of 2.5 mm.
Given the layer capacitors under discussion here and which comprise plastic films as a dielectric, the spacing of wire-like power lead elements is defined by the width of the metallized plastic tapes to be wound onto the drum in combination with the metal layers (end contact layers) disposed at the opposite sides, such as disclosed, for example, in U.S. Pat. Nos. 3,170,211 and 3,693,244, incorporated herein by reference. For reasons that shall be presented in greater detail later in conjunction with the description of the figures, it was previously only possible to obtain layer capacitors having a smallest grid dimension of 5 mm.
Given the manufacture of the layer capacitors under discussion here and comprising plastic films as dielectric, metallized plastic tapes, which are alternately respectively provided with metal-free strips at opposite edges, are wound onto the drum. A curved mother capacitor corresponding to the radius of the winding wheel (usually 25 through 50 cm) or an originating capacitor arises given a plurality of mother capacitors wound onto the wheel and which is provided with metal layers at its end faces by means of metal spraying (Schoop's process, for example according to the U.S. Pat. No. 1,128,058, incorporated herein by reference). After the division of the originating capacitor into mother capacitors, individual capacitors are sawed from these. Various techniques are known to insure that a greater insulation spacing results between the antipolar metal coatings at the sawed surface than corresponds to the thickness of the dielectric layers (15 .mu.m down to 1 .mu.m). Thus, the German Letters Pat. No. 17 64 548 (corresponds to U.S. Pat. No. 3,614,561, incorporated herein by reference), proposes the employment of stretched plastic films for this purpose. The sawing is undertaken with a topical heating of the plastic films (without the application of a burn-out voltage) such that a small edge strip of the synthetic is thus provided.
In order to achieve adequate insulation in the region of the cut edge, the German Letters Pat. 17 64 549 (corresponding to U.S. Pat. No. 3,590,347, incorporated herein by reference) proposes that at least one part of the dielectric film be provided with solvent at least in the region of the cut surfaces before the thermal treatment, and be further treated such that the solvent component amounts to about 0.25% there during the thermal treatment. Thus, a collapse of the continuity of the metal coating is achieved in the edge regions.
The German Letters Pat. No. 25 26 130 (corresponding to U.S. Pat. No. 4,041,587, incorporated herein by reference), proposes that the saw blade be coated during the sawing process with an insulating lubricant since the mother capacitor to be divided is provided with a polyolefin film that melts due to the heat developed when sawing. This film is sawed together with the mother capacitor and the saw blade first penetrates the polyolefin films in the region of its engagement, and only then penetrates the capacitively effective region of the capacitor.
In order to enhance the contacting between the metal layers applied to the end faces and the equipolar coatings on each and every side, the German Letters Pat. No. 24 16 566, incorporated herein by reference, discloses an electrical capacitor, particularly a stack or layer capacitor, that is end-contacted with the Schoop process and which contains metallized capacitor films consisting of insulator layers and metallizations whose end-contacted edges have a wavy path. Accordingly, the overall length of the edges only amounts to a few longitudinal waves of the waviness and the frequency of the waviness of the edges of adjacent film plies is different.
The references described above with respect to layer capacitors having a plastic dielectric shows that capacitors always result in which the current path proceeds from one end contacting, via the metal coatings and through the dielectric (displacement currents) onto the counter-coatings of the same directions, and from there to the other end contact layer, as shown in FIG. 7. These currents are not compensated and generate a magnetic field.
Such a current path likewise fundamentally applies for so-called capacitative networks such as disclosed in the German Pat. No. 17 64 861 (corresponding to Great Britain Letters Pat. No. 1,220,567, incorporated herein by reference). These capacitors are manufactured according to the method for manufacturing layer capacitors in the fashion described above and are subsequently provided with deep incisions so that internal series connections result.
The simplest form of such a capacitative network having an internal series connection is shown in FIGS. 2 and 3 of German Letters Pat. No. 1,940,036 (corresponds to British Pat. No. 1,289,206, incorporated herein by reference). These figures show two dielectric films from a capacitor which is practically designed in U-shaped fashion due to the incision. The one upper dielectric ply comprises a metal coating which does not extend up to the edge at any side of the U-shape, and which itself is in turn U-shaped.
The other, lower dielectric ply comprises two metal coatings which are only situated on the legs of the U-shape and extend up to the ends of the two U-legs. Such an arrangement of the coatings likewise leads to a current path that is not compensated.
Apart from this, these capacitors are not primarily intended for employment in printed circuits.
An increasingly greater expense for the interference portection and for the stabilization of the supply voltages is required in the field of modern electronics due to the higher and higher operating frequency. Anti-interference capacitors consisting of metal films and a bifilar winding are known. However, they are too large and too expensive for the required uses. Very small ceramic multi-layer capacitors, likewise having a bifilar format, have been recently disclosed, as explained with reference to the two references "IBM Technical Disclosure Bulletin" cited above. These are matched to the demands of modern electronics in terms of their capacitance range and their dimensions. As explained, the manufacturing method for such capacitors is complicated and therefore unsuited for mass production.